Washington ? A simple foam paint brush that costs only pennies at hardware stores could be an essential tool in returning the space shuttle to orbit, NASA’s administrator said Wednesday.

Space agency engineers found that the brush may be just what astronauts need to spread a patching compound on a space shuttle’s damaged heat shield while the craft is in orbit.

“This thing turns out to be one of the most valuable tools we could have invented,” said Sean O’Keefe, head of the National Aeronautics and Space Administration. “We’re going to buy it at Wal-Mart. We’re not going to ask the Defense Department to requisition it out of stock.”

A clerk at a Washington-area hardware store said a 1-inch foam brush sells for 49 cents and a 2-inch one cost 99 cents.

Designing and testing a way to repair damage in the shuttle’s heat shield is an important part of NASA’s efforts to return the space shuttle to orbit after the Feb. 1 accident that destroyed Columbia and killed seven astronauts.

Astronauts on Columbia and engineers in Mission Control were not aware of the extent of damage to the shuttle wing. But officials said that, in any case, there was no equipment on the shuttle to patch the wing even if the problem were recognized.

Officials at the Johnson Space Center said the patching compound now under consideration could be used only for the part of the heat shield composed of lightweight tiles. The nose of the shuttle and the leading edge of the wings are covered with panels of a different material, reinforced carbon-carbon. It was a break in a carbon-carbon panel that led to the loss of Columbia.

Before returning to the shuttle to space, the investigation board said NASA must develop a way for astronauts to fix heat shield damage while the craft is in orbit.

Such work would require a difficult spacewalk, and the damage would require patching with a substance that could withstand the 3,000-degree heat of re-entry.

To repair the heat shield, O’Keefe said engineers have found a sealant that is formed when two compounds are mixed together. Tests showed that the combined compound expanded when heated. This led to a plan to “underfill” a hole and then let the heat of re-entry swell the patch and seal the hole.

Once they had a compound, the engineers then had to find a way for a spacewalking astronaut to apply the material while wearing a bulky space suit, gloves and a bubble helmet, O’Keefe said.

Experts looked at a variety of sophisticated tools made from exotic materials, he said. They settled on what he called “an elegant piece of sophisticated hardware” — a foam paint brush with a wooden handle.